This invention relates generally to improvements in controlling silicon crystal growth processes and, particularly, to a vision system and method for measuring melt level in a Czochralski silicon crystal growth process for use in controlling the growth process.
Single crystal, or monocrystalline, silicon is the starting material in most processes for fabricating semiconductor electronic components. Crystal pulling machines employing the Czochralski process produce the majority of single crystal silicon. Briefly described, the Czochralski process involves melting a charge of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace. After the silicon in the crucible is melted, a crystal lifting mechanism lowers a seed crystal into contact with the molten silicon. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt.
After formation of a crystal neck, the typical process enlarges the diameter of the growing crystal by decreasing the pulling rate and/or the melt temperature until a desired diameter is reached. By controlling the pull rate and the melt temperature while compensating for the decreasing melt level, the main body of the crystal is grown so that it has an approximately constant diameter (i.e., it is generally cylindrical). Near the end of the growth process but before the crucible is emptied of molten silicon, the process gradually reduces the crystal diameter to form an end cone. Typically, the end cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable, or shaft, along with the seed and the crystal in an opposite direction.
The Czochralski process is controlled in part as a function of the level of molten silicon in the crucible. Thus, an accurate and reliable system for measuring melt level during the different phases of crystal growth is needed to ensure crystal quality. Commonly assigned U.S. Pat. Nos. 5,665,159 and 5,653,799 and U.S. application Ser. No. 08/896,177 (allowed), the entire disclosures of which are incorporated herein by reference, provide accurate and reliable measurements of a number of crystal growth parameters, including melt level. In these patents, an image processor processes images of the crystal-melt interface to determine the melt level.
U.S. Pat. Nos. 3,740,563 and 5,286,461, the entire disclosures of which are incorporated herein by reference, also disclose means for measuring melt level. A moving, closed loop electro optical system provides a melt level measurement in U.S. Pat. No. 3,740,563 and detection of a reflected laser beam provides a melt level measurement in U.S. Pat. No. 5,286,461.
Although presently available Czochralski growth processes have been satisfactory for growing single crystal silicon useful in a wide variety of applications, further improvements are still desired. For example, hot zone apparatus are often disposed within the crucible to manage thermal and/or gas flow. For control purposes, it is often desirable to measure the melt level relative to the hot zone apparatus and to measure the position of different hot zone parts relative to each other.
One known method predicts the position of a reflector, for example, based on the "stack up" of the dimensions and tolerances of multiple supporting parts. However, most of these parts are susceptible to thermal expansion and, thus, the actual position of the reflector is not known to the accuracy required for satisfactory product quality. Another common practice is to suspend a quartz pin of known length from the reflector. Moving the crucible until the melt touches the pin establishes the position of the reflector with respect to the melt. However, this incurs additional fabrication costs, introduces another process step and requires greater diligence during puller setup and cleanup to correctly install and use the pin without damaging it.
U.S. Pat. No. 5,437,242, the entire disclosure of which is incorporated herein by reference, discloses directly determining the distance between the reflector and its reflection in the melt. Unfortunately, the method of this patent is incapable of providing the reflector position. Also, this method requires a mechanical reference mark in the reflector having the shape of, for example, a triangle, quadrangle or circle. In this instance, the mark obscures the view of the far edge of the reflector and its reflection and incurs additional fabrication costs. Using a mechanical reference mark in the reflector also requires greater diligence during puller setup to align the mark correctly and affects the thermal and gas flow properties of the reflector itself.
For these reasons, an improved system and method for the measurement and control of melt levels and the positions of hot zone parts in the Czochralski process, without additional setup procedures and processing steps and without additional consumable parts, is desired.